1. Field of the Invention
The present invention relates to image correction technology for improving image quality declined due to nozzles suffering ejection failure in an inkjet image forming apparatus.
2. Description of the Related Art
In the field of inkjet image formation, various measures are adopted in order to achieve image formation of high resolution by means of an inkjet head; for example, as shown in FIG. 13, a head 300 is constituted by a structure in which a plurality of nozzle head modules 301 are arranged in a staggered configuration, and the recording position pitch Δx on the paper 340 (image receiving medium) is made narrower than the pitch Pm of the nozzles 320 in the head module 301, thereby raising the recording resolution, and so on. In the example in FIG. 13, a head 300 is composed so as to have a nozzle arrangement (staggered arrangement) whereby the recording position pitch Δx on the paper 340 is approximately Pm/2.
By conveying the paper 340 in a substantially perpendicular direction to the lengthwise direction of the head 300 at a uniform speed and controlling the droplet ejection timing of the nozzles 320, it is possible to form a desired image on the paper 340. Here, it is supposed that the paper 340 is conveyed from the lower side toward the upper side in FIG. 13. If the conveyance direction of the paper 340 is the y direction and the width direction of the paper perpendicular to this is the x direction, then it is possible to form dots (recording points formed by depositing liquid droplets) at a pitch of Δx in the x direction on the paper 340. Here, Δx is a value which corresponds to the recording resolution (in the case of 1200 dpi, approximately 21.2 μm).
The alignment sequence of nozzles 320 capable of forming a dot row in the x direction on the paper 340 at a pitch (Δx) corresponding to the recording resolution (the alignment sequence of nozzles obtained by projecting the nozzle arrangement in the head 300 onto the x axis) gives the effective nozzle arrangement. In the present specification, nozzles which are in a mutually adjacent positional relationship in the nozzle alignment sequence of this effective nozzle row (the projected nozzle row on the x axis) are called “adjacent nozzles”. In other words, nozzles which are not necessarily in adjacent positions in the nozzle layout in the head 300 but are aligned in adjacent positions when viewed as a projected nozzle row on the x axis of the paper 340, are called “adjacent nozzles”.
When an inkjet head of this kind is installed in a printing apparatus, it is necessary to adjust the angle and position of installation of the head, but there are limits on the mechanical adjustment precision. Consequently, there are cases where, as shown in FIG. 14, the head 300 is slightly rotated from the specified position (the ideal installation position according to the design) and where the head 300 is installed on the printing apparatus in a state having a residual amount of rotation (Δθ). Furthermore, there are also cases where, as shown in FIG. 15, the arrangement positions of the head modules 301 are slightly divergent and the head 300 is installed on the printing apparatus in a state having a residual divergence in the arrangement position (Δd). When ink is ejected from the nozzles 320 of the head 300 in a state of this kind, error (“depositing position error”) occurs in the depositing positions on the paper 340.
Furthermore, in addition to the problem of depositing position error caused by the adjustment accuracy described above, when starting to use the inkjet head, nozzles which are in a state of ejection failure also arise due to blockages, failures, and the like. In particular, in the case of image formation by a single pass method, the ejection failure nozzle locations are perceived as white stripes and therefore must be corrected. Various different ejection failure correction technologies for improving image defects arising due to ejection failure nozzles of this kind have been proposed in the related art (see, for example, Japanese Patent Application Publication No. 2007-160748). The basic approach to ejection failure correction technology is to improve visibility by adjusting the output image density or the ejected dot size in a plurality of nozzles before and after an ejection failure nozzle.
When general ejection failure correction technology is used in a head having depositing position errors and ejected droplet volume errors, if the same correction coefficient is used for all of the ejection failure nozzles, then correction may be excessive or insufficient, depending on the state of arrangement of the nozzles, and black stripes or white stripes may become visible on the surface of the paper.
FIG. 16 illustrates schematic views of this phenomenon in (a) to (d). Here, as described using FIG. 14, a case is described by way of example in which a head 300 is installed with a residual amount of rotation (Δθ), and an upper-stage nozzle NA_j and a lower-stage nozzle NB_k are ejection failure nozzles which are suffering an ejection failure (see (a) of FIG. 16). In this case, the ejection failure correction technology of the related art corrects the values (image setting values representing density tone graduations) of pixels corresponding to nozzles which are adjacent before and after the ejection failure nozzles (before and after the ejection failure nozzles in the alignment sequence of the effective nozzle row). In (a) to (d) of FIG. 16, the image setting values of the positions corresponding to the adjacent nozzles NB_j−1 and NB_j+1 before and after the ejection failure nozzle NA_j are corrected, and furthermore the image setting values of the positions corresponding to the adjacent nozzles NA_k−1 and NB_k+1 before and after the ejection failure nozzle NB_k are corrected.
(b) of FIG. 16 shows a schematic view of a state where a solid image (uniform density image) having a certain density (tone value) is formed by using a general ejection failure correction technology in the head 300 in (a) of FIG. 16. Since dots cannot be formed at the positions corresponding the ejection failure nozzles NA_j, NB_k on the paper (the positions in the x direction), then the prescribed density cannot be achieved in the corresponding portions of the image. In order to compensate for this, correction is performed to increase the output density of the adjacent nozzles. (c) of FIG. 16 shows the image setting values of the pixels corresponding to respective nozzle positions. In the case of a tone value D1 indicating a density of a solid image, correction is performed to amend the image setting values to a higher value (D2) using a prescribed correction coefficient in positions which correspond to the adjacent nozzles of the ejection failure nozzles.
However, taking a macroscopic view of the output results after correction, the position corresponding to the ejection failure nozzle NA_j on the paper is over-corrected, the output density becomes high and a so-called “black stripe” appears, as shown in (d) of FIG. 16. Furthermore, the position corresponding to the ejection failure nozzle NB_k is under-corrected, the output density is low and a so-called “white stripe” appears.
In respect of phenomena of this kind, Japanese Patent Application Publication No. 2007-160748 seeks to overcome the aforementioned problem by calculating separate correction coefficients for each nozzle from the depositing position error and the ejected droplet volume error. Furthermore, in many methods, the correction performance for each image setting value is raised by preparing a correction coefficient reference table for ejection failure correction (hereinafter, called a “correction LUT”) in respect of the image setting value (image density/image tone) with respect to each nozzle.
However, in the ejection failure correction technology of the related art, the physical conditions which are considered in particular as the dominant factors are mainly limited to two items only, namely, the depositing position and the dot diameter (which has a correlation with the volume of the ejection droplet) of the ejected liquid. The image formation process by an inkjet head cannot be described fully on the basis of these two physical conditions alone, and there are also cases where sufficient correction performance is not obtained with related art correction technology which only considers these two items. One example of a dominant factor which is not considered in ejection failure correction technology of the related art is “landing interference”. This landing interference is a phenomenon which occurs when adjacent dots contact each other and combine together. Landing interference is a phenomenon which is closely linked to the depositing positions and the dot diameter. For example, even in a state where the depositing position error is the same, the presence or absence of landing interference varies depending on the size of the dot diameter. Furthermore, the presence and absence of landing interference also varies in a similar fashion in cases where the dot diameter is the same but there is change in the degree of the depositing position error.
Moreover, the presence or absence of landing interference also varies with the time difference of droplet ejection between the peripheral dots, in other words, the deposition sequence. FIGS. 17A and 17B are schematic drawings for describing the presence or absence of landing interference depending on the deposition sequence. FIGS. 17A and 17B assume an ideal state where the depositing position error and the dot diameter of the nozzles 320 in the head 300 described in relation to FIG. 13 are the same in all of the nozzles, and show a case where there is a nozzle of the nozzles in this head 300 which has suffered ejection failure.
FIG. 17A shows a case where one nozzle NB_k has suffered ejection failure, of the nozzle row situated to the upstream side of the paper conveyance direction in the head 300 (in FIG. 13, the lower-stage nozzle row; hereinafter “upstream nozzle row”). In the head 300 in FIG. 13, ejection is performed firstly from the upstream nozzle row which is situated on the upstream side in terms of the conveyance direction of the paper 340, whereupon ejection is performed from the nozzle row on the downstream side (the upper-stage nozzle row in FIG. 13).
In other words, there is a time difference between droplet ejection from the upstream nozzle row and the downstream nozzle row (in other words, a deposition time difference). The left-hand side diagram in FIG. 17A shows a state where a liquid droplet 350B ejected from a nozzle in the upstream nozzle row reaches the surface of the paper 340 before a liquid droplet 350A ejected from a nozzle in the downstream nozzle row. If the nozzle NB_k belonging to the upstream nozzle row is suffering an ejection failure, then no liquid droplet is present on the position on the surface of the paper corresponding to the ejection failure nozzle NB_k. In FIG. 17A, an ejection failure is indicated by a broken line.
In this case, the droplets 350A_k−1 and 350A_k+1 ejected from the nozzles adjacent to the ejection failure nozzle NB_k (hereinafter, a nozzle adjacent to an ejection failure nozzle is called an “adjacent to ejection failure nozzle”) aggregate with the droplets 350B_k−2 and 350B_k+2 ejected previously by adjacent nozzles further to the outside. The depositing position error of an adjacent to ejection failure nozzle is increased by this aggregating action (landing interference), and the droplet ejection pitch (pitch between dots) before and after the ejection failure nozzle NB_k is increased. More specifically, the pitch ΔSA between dots formed by droplets ejected by a pair of adjacent to ejection failure nozzles becomes greater (see the right-hand figure in FIG. 17A).
On the other hand, FIG. 17B shows a case where one nozzle NA_j has suffered ejection failure, of the nozzle row situated to the downstream side in terms of the paper conveyance direction in the head 300 shown in FIG. 13 (in FIG. 13, the upper-stage nozzle row; hereinafter “downstream nozzle row”).
In this case, the liquid droplets 350B_k−2 and 350B_k+2 which are ejected by the adjacent nozzles (adjacent to ejection failure nozzles) before and after the ejection failure nozzle NA_j are deposited first on the paper surface, and therefore an aggregating action (landing interference) as described above does not occur. Therefore, the droplet ejection pitch (pitch between dots) before and after the ejection failure nozzle NA_j is narrower than in the case of FIG. 17A. In other words, the pitch ΔSB between the dots formed by droplets ejected by the pair of adjacent to ejection failure nozzles becomes narrow as shown on the right-hand side in FIG. 17B (ΔSB<ΔSA).
In FIGS. 17A and 17B, the droplets (dots) deposited on the paper surface are depicted as having a spherical shape, but this is for the sake of simplicity in order to clarify the relationship between the ejected droplets 350A and 350B, and in actual practice the deposited droplets (dots) have a shape which spreads over the paper surface at an angle of contact that is defined by the properties of the liquid and the surface properties of the paper surface.
As described above, even in an ideal case where the depositing position error and the dot diameter of the nozzles 320 in the head 300 shown in FIG. 13 are the same in all of the nozzles, the positional error can increase depending on the deposition sequence, the droplet ejection pitch before and after the ejection failure nozzle can become larger or smaller, and the visibility of the stripes can vary greatly.
In this way, in image formation by an inkjet head, it is not possible to ignore the effects of landing interference. The ejection failure correction technology is also affected by these factors. In Japanese Patent Application Publication No. 2007-160748, the depositing position error of each nozzle is calculated in advance, but in this measurement, it is necessary to create conditions where no image formation is performed in the vicinity of the nozzle for which the position error is to be measured, and landing interference does not occur.
However, when actually performing image formation, as shown in FIGS. 17A and 17B, landing interference occurs and therefore the measurement value of the position error measured under conditions where landing interference does not occur diverges greatly from the actual value. Consequently, correction technology using a general technique which considers only depositing position error and ejected droplet volume error can produce results in which a combination of blank stripes and white stripes are visible on the surface of the paper ((d) of FIG. 16).